Silicate precursors, such as tetramethoxysilane (Si(OCH3)4, “TMOS”), are commonly used to produce porous silica glasses (Brinker et al., Sol-Gel Science, Academic Press, New York (1990)). The resultant materials are highly crosslinked polymer matrices with solvent filled pore spaces. This solvent can be evacuated from the pore-matrix either by evaporation or by supercritical extraction; the two methods yield materials with different physical properties. During solvent evaporation, surface tension, which exists at any liquid-vapor interface, exerts a force large enough to collapse the pore structure until the gel network becomes strong enough to resist this compressive force (Brinker et al., Sol-Gel Science, Academic Press, New York (1990)). This process generates a condensed silicate matrix, referred to as a xerogel, which is made up of 60-90% air and has pore diameters of 1-20 nm (Brinker et al., Sol-Gel Science, Academic Press, New York (1990)). Evacuating the solvent above its critical point, where neither liquid nor gas is present, can eliminate the surface tension. Consequently, the pore structure does not collapse, but is instead maintained, yielding a low-density solid known as an aerogel (Pierre et al., Chem. Rev. 102:4243 (2002)). Aerogels typically consist of 90-99% air (Lev et al., J. Gun, Anal. Chem. 67:22A (1995)), with pore diameters from 1-50 nm (Brinker et al., Sol-Gel Science, Academic Press, New York (1990)).
The resulting aerogel structure is responsible for giving aerogel materials claim to the lowest known density, index of refraction, thermal, electrical, and acoustical conductivities of any solid material. First discovered by Kistler (J. Phys. Chem. 63: 52 (1932)) in the 1930s, many attempts have been made to take advantage of their unique properties. Current application areas include Cerenkov radiation detectors (Ganezer et al., IEEE Trans. Nucl. Sci. 41:336 (1994); Hasegawa et al., Nucl. Inst. Phys. Res. 342:383 (1994)), electronics (Hrubesh et al., J. Non-Cryst. Solids 188:46 (1995)), thermal insulators (Reiss, Phys. Blaetter 48:617 (1992)), insulated windows (Hrubesh et al., J. Non-Cryst. Solids 188:46 (1995); Lampert, Int. J. Energy Res. 7:359 (1983)), comet dust collectors (Tsou, J. Non-Cryst. Solids 186:415 (1995)), and heat storage devices for automobiles (Fricke et al., J. Sol-Gel Sci. Technol. 13:299 (1998)).
At present, aerogel materials are difficult and expensive to manufacture. It can take days to weeks to make an unbroken monolith. This manufacturing complexity has limited their development in commercial applications. Aerogels are typically formed in a two-step process. The first step is to form a wet gel by a sol-gel polymerization reaction. The second step is to extract the solvent and dry the wet gel to form the aerogel.
The primary challenge in the fabrication of an aerogel is to prevent the collapse of the porous structure during the drying phase. Stress contributors such as thermal gradients and pressure concentrations are significant in aerogel fabrication, but relatively easy to minimize. More difficult to control, however, are the capillary stresses from surface tension that, for the nanoscale pore structure, are strong enough to cause structural collapse. As the sol-gel dries, these capillary forces can result in significant fracture to the structure. The current methods used to avoid fracture in aerogel fabrication can be categorized into three general techniques, although each drying protocol is designed to minimize or eliminate surface-tension effects. They are (1) ambient pressure techniques; (2) conventional supercritical extraction (CSCE) techniques; and (3) rapid supercritical extraction (RSCE) techniques.
The ambient-pressure techniques attempt to dry the wet gel at ambient pressure. To do so they require chemical processes to reduce the capillary forces. One method is to treat the surface of the gel with a surfactant, or surface-tension-reducing chemical (see, e.g., Yusuf et al., J. Non-Cryst. Solids 285:90 (2001); or Lev et al., Anal. Chem. 67:22A (1995)). Another technique used by Hereid et al. (J. Sol-Gel Sci. Technol. 3:199 (1994)) ages the gels in alkoxide/alcohol solutions to stiffen the microstructure and avoid collapse due to capillary forces. A technique developed by Prakash et al. (J. Non-Cryst. Solids 190:264 (1995)) manipulates the surface chemistry of the gel to aid in the solvent evacuation. This method uses a solvent exchange with hexane, followed by a surface modification with a silylation process to promote a reversible shrinkage. These techniques have been used successfully in the fabrication of aerogel films, but have had limited success for aerogel monoliths.
The conventional supercritical extraction techniques (CSCE) are multi-step techniques designed to eliminate surface tension altogether by bringing the sol-gel to the critical point of the solvent. Above the critical point there is no surface tension, and the solvent can be evacuated without damage to the gel structure. The technique first developed by Kistler (J. Phys. Chem. 63: 52 (1932)) entailed two steps: the formation of the wet gel, and the subsequent solvent evacuation in a heated pressure vessel at the supercritical conditions. The main limitations of this technique are the difficulties associated with obtaining the high temperatures necessary to reach the critical point of the alcohol solvent, as well as the safety concerns with operating the pressure vessel at those conditions. In response to these concerns, a lengthy solvent exchange with liquid CO2 can be performed prior to supercritical extraction, which can then take place at the critical point of CO2 (see, e.g., Tewari et al., in Aerogels, J. Fricke (Ed.), Springler-Verlag, New York (1986), p. 31; Van Bommel et al., J. Non-Cryst. Solids 186:78 (1995)). The advantages of the CSCE method are a lower critical temperature and a more stable solvent; however an additional step is added to the process. Because the critical pressure requirement is not changed significantly, this process still requires the use of thick pressure vessels and places practical limitations on the maximum size of the aerogel. In addition, the solvent-exchange process becomes a size deterrent, as the diffusion kinetics of the solvent exchange depend upon the size of the gel. Even if a pressure vessel were available to contain a large monolith, the solvent exchange could take weeks to complete, depending on the size of the monolith.
The third technique, rapid supercritical extraction (RSCE), was developed by Poco et al. (Mat. Res. Soc. Symp. 431:297 (1996)) and described further in Scherer et al. (J. Non-Cryst. Solids 311:259 (2002)). Similar to the CSCE techniques, RSCE is a supercritical technique designed to perform the solvent extraction under supercritical conditions. In contrast to the CSCE techniques, however, the RSCE is a one-step, reactant-to-aerogel process. The liquid precursor chemicals and catalyst are inserted into a two-piece mold that is then heated rapidly to speed up the polymerization. The pressure is initially set by fastening the two mold parts together with properly tensioned bolts, or by applying an external hydrostatic pressure inside of a larger pressure vessel, or by a combination of these two. Once the supercritical point of the alcohol is reached, the supercritical fluid is allowed to escape through gaps formed by the roughness in the surface contact between the two portions of the mold, or through a relief valve set just above the supercritical pressure. A benefit of this method is that the entire process is done in one step, and can be accomplished in under an hour, as opposed to multiple steps (and time scales on the order of weeks) for all other available methods.
The advantage of the ambient-pressure methods is that they do not require expensive and potentially dangerous pressure equipment. They are currently being used successfully in the fabrication of aerogel powders and thin films. For the fabrication of monolithic pieces, however, this technique has yet to prove reliable. Conventional supercritical extraction has been used extensively in the fabrication of very large aerogel monoliths, however it can take days to weeks to make them, and the required multiple steps make the process complicated. In addition to the reduction in fabrication time, the rapid supercritical extraction as a one-step process has the most potential for reliable and repeatable fabrication, as well as increased production volume.
The present invention relates to a fast supercritical extraction technique for fabricating aerogels that overcomes the above-identified deficiencies of the prior art.